Inner and outer surfaces of metal cans generally have coatings for the protection against corrosion. For simplifying the manufacturing step, improving the sanitary condition and preventing the environmental pollution, however, there have been recently developed methods of imparting rustproof properties to metal cans without using any organic solvent. One of the methods is to coat metal cans with a film of a thermoplastic resin. That is, studies have been being made of a method in which a plate of tin-plated steel, tin-free steel or aluminum is laminated with a film of a thermoplastic resin and the resultant laminate is deep-drawn to make cans. Attempts have been made to use a polyolefin film or a polyamide film as the above film of a thermoplastic resin, but not all of moldability, heat resistance, flavor retaining property and impact resistance are satisfied.
On the other hand, a polyester film, or a polyethylene terephthalate film in particular, is drawing attention as one having well-balanced properties, and several proposals have been made to use it as a base film as follows.
(A) A metal plate is laminated with a biaxially oriented polyethylene terephthalate film through an adhesive layer of a polyester having a low melting point, and the resultant laminate is used as a material for making cans (see Japanese Laid-open Patent Publications Nos. 10,451/1981 and 192,546/1989).
(B) A metal plate is laminated with a film of an aromatic polyester having amorphous nature or very low crystallinity, and the resultant laminate is used as a material for making cans (see Japanese Laid-open Patent Publications Nos. 192,545/1989 and 57,339/1990).
(C) A metal plate is laminated with a heat-set, biaxially oriented polyethylene terephthalate film having a low orientation degree, and the resultant laminate is used as a material for making cans (see Japanese Laid-open Patent Publication No. 22,530/1989).
Concerning (A), the biaxially oriented polyethylene terephthalate film is excellent in heat resistance and flavor retaining property, while it is insufficient in moldability so that it is whitened (causes fine cracks) or broken during the can-making processing which entails large deformation.
Concerning (B), the film used is an amorphous or very low crystalline aromatic polyester film and therefore has excellent moldability, while the film is poor in flavor retaining property and is liable to embrittle when printing is effected on the film, cans are post-treated for retort treatment or cans are stored for a long period of time, and the embrittled film is liable to break due to an external impact.
Concerning (C), the laminate is to produce an effect in a region between (A) and (B), while the film has not yet attained the low orientation degree which can be applied to can-making processing. Further, even if the laminated is moldable in a region where the degree of deformation is small, the film is liable to embrittle when printing is thereafter effected or when the can is subjected to retort treatment for sterilizing canned contents, and the embrittled film is liable to break due to an external impact, as discussed concerning (B).
Further, the lubricity and abrasion resistance of a polyester film are main factors which have an influence on the processability in the step of film production and the step of laminate processing for various uses and on the product quality.
That is, when the lubricity is insufficient, a crease is liable to occur on the film in the step of taking up the film, or a crease is liable to take place on the film when the film is laminated on a metal. When the abrasion resistance is insufficient, pinholes are liable to occur on the film in the drawing step for making cans, and the film breaks in an extreme case. Thus, the film does not serve as a coating on inner and outer surfaces of a metal can.
For improving a magnetic recording film in lubricity and abrasion resistance, there is generally employed a method in which the film is imparted with an uneven surface by incorporating inert fine particles, so that the contact area of the film to a roll and a processing tool decreases. Generally, the larger the size of the fine particles in a polymer as a raw material is, the greater the effect on the improvement in lubricity. It is well known practice to incorporate inert fine particles of calcium carbonate, titanium dioxide or kaolin, or inert fine particles of at least two of these (combination of particles having a greater size and particles having a smaller size) (see Japanese Laid-open Patent Publications Nos. 34,272/1976, 78,953/1977, 78,954/1977, 41,355/1978 and 71,154/1978).
However, in the step of a drawing process for making cans which involves a large deformation, as the size of the fine particles increases, the size of voids formed in the interface between each fine particle and a polyester at the time of deformation increases, and the projections of the fine particles are altered into moderate shapes to increase a frictional coefficient at the time of processing. At the same time, some particles may drop off due to small scratches formed on the voids, which may cause the occurrence of pinholes or the breakage of the film.
It is an object of the present invention to provide a laminated polyester film for use as a film to be laminated on a metal plate, which retains excellent moldability, heat resistance, resistance to embrittlement under retort treatment and flavor retaining property of a copolyester film and further has other excellent performances.
It is another object of the present invention to provide a laminated polyester film for use as a film to be laminated on a metal plate, which is improved in impact resistance, particularly, which is almost free from having cracks when an impact is exerted at a low temperature.
It is further another object of the present invention to provide a polyester film for use as a film to be laminated on a metal plate, which is free from affecting the tastes of refreshing beverages and excellent in flavor retaining property.
Other objects and advantages of the present invention will be apparent from the following description.